1. Field of the Invention
The present invention relates to a method for synthesizing an anthracene derivative.
2. Description of the Related Art
In recent years, research and development of light-emitting elements using electroluminescence (EL) have been actively carried out. In a basic structure for these light-emitting elements, a layer containing a luminescent substance is interposed between a pair of electrodes. By applying a voltage to this element, luminescence can be obtained from the luminescent substance.
Such light-emitting elements are classified into a self-luminous type, and thus have advantages such as higher pixel visibility and the eliminated need for a backlight, as compared with liquid crystal displays. Accordingly, such light-emitting elements are thought to be suitable as flat panel display elements. In addition, such light-emitting elements also have the great advantage of being able to be manufactured to have thinness and lightness. Furthermore, it is also one of the features of such light-emitting elements that the response speed is very high.
Furthermore, since these light-emitting elements can be formed in the form of a film, planar light emission can be obtained. Therefore, large-area elements can be easily formed. This is a feature which is difficult to obtain from point light sources typified by an incandescent lamp and an LED or linear light sources typified by a fluorescent lamp. Accordingly, the light-emitting elements using EL have a great deal of potential for use as planar light sources which can be applied to illumination and the like.
The light-emitting elements using electroluminescence can be roughly classified in accordance with whether the luminescent substance is an organic compound or an inorganic compound. In the case of an organic EL element in which an organic compound is used for the luminescent substance to provide a layer containing the luminescent organic compound between a pair of electrodes, when a voltage is applied to the light-emitting element, electrons and holes are respectively injected from a cathode and an anode into the layer containing the luminescent organic compound, thereby allowing a current to flow. Then, the injected electrons and holes bring the luminescent organic compound into an excited state, in such a way that luminescence is obtained from the excited luminescent organic compound.
On the basis of such a mechanism, the above-described light-emitting element is referred to as a current-excitation type light-emitting element. It is to be noted that the excited states formed by an organic compound include a singlet excited state and a triplet excited state, and luminescence from the singlet excited state is referred to as fluorescence, whereas luminescence from the triplet excited state is referred to as phosphorescence.
In addition to luminescence through recombination of carriers excited by a current, there is also a method in which the excitation energy of an organic compound excited by a current is transferred to another organic compound, thereby exciting the latter organic compound to produce luminescence. This method is effective in the case of the luminance efficiency reduced (concentration quenching) due to stacking interaction caused by a high concentration of organic molecules that are desired to produce luminescence. In organic EL elements, the method is generally applied to the element structure used in which a luminescent material is dispersed in a light-emitting layer (a light-emitting layer is doped with a luminescent material). Doping a host material with organic molecules that are desired to produce luminescence suppresses the stacking interaction, thereby allowing the efficiency of the light-emitting element to be increased. In such a light-emitting element, the excitation energy is transferred from a host material excited by current excitation to a dopant material, thereby allowing the dopant material to produce luminescence. It is to be noted that when a substance A is dispersed in a matrix composed of a substance B, the substance B constituting the matrix is referred to as a host material, whereas the substance A dispersed in the matrix is referred to as a dopant material.
While various analyses have been carried out in regard to the deterioration mechanism of light-emitting elements, the mechanism has not been fully determined yet actually. It is believed that the deterioration mechanism involves various factors, and the purity of an organic material for use in light-emitting elements can be cited as one of the factor. As for a light-emitting element, it is known that if an inorganic metal compound is adjacent to a light-emitting site of the light-emitting element, the light-emitting site is changed to a quenching site, which results in one of the factors that the efficiency of the element is decreased or that the element is deteriorated. Therefore, low molecular weight organic materials for organic EL elements are typically purified before use by a method such as sublimation to increase the degree of purity, in addition to common methods for purification of organic matters. However, in regard to the deterioration factors caused by the other impurities, it has not been clear yet which factor is involved in deterioration. Therefore, in order to clarify the deterioration factors as much as possible, it has been desired to obtain materials including fewer impurities.
Furthermore, in general, it is desirable in regard to the synthesis of organic compounds to obtain intended products with a higher degree of purity in a simple manner, and various methods have been thus made. The means for obtaining intended products with a higher degree of purity in a simple manner include the adoption of a synthesis root using more stable raw materials which are easily purified and the adoption of a synthesis root which is less likely to synthesize by-products. As described above, efforts have been made such that the lot-to-lot variation in material can be further reduced. When materials with reduced lot-to-lot variation in material are adopted for devices such a light-emitting elements, characteristics with less variation can be obtained.
In addition, while organic compounds are able to be synthesized in a variety of ways, the synthesis often involves multiple synthesis steps. Therefore, the more complex the synthesis method is, the more raw materials and time are consumed. Accordingly, the proposal of simpler synthesis methods has been desired.
One of organic materials for use in light-emitting elements is 9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: PCzPA) (Patent Document 1). This material is a material which is stable against the repetition of reduction and oxidation states (the repetition of from the oxidation state to the reduction state and from the reduction state to the oxidation state), and in particular, can be preferably used as a hole injection layer, a hole transport layer, and a light-emitting layer (a host or a dopant, a luminescent material). However, the method for synthesizing an anthracene derivative such as PCzPA, described in Patent Document 1, has difficulty in removal of a by-product which is likely to be produced in the process. Therefore, if this material can be produced with a high degree of purity and with less variation in a simple manner, more desirable characteristics are obtained stably in light-emitting elements, etc.